Cherry-pick of 5d671a8 onto stable5.

recreate an etag within the scanner if the cache contains an empty etag

Conflicts:
	lib/files/cache/cache.php
	tests/lib/files/cache/scanner.php
Cherry-pick of 5d671a8 onto stable5.

recreate an etag within the scanner if the cache contains an empty etag

Conflicts:
	lib/files/cache/cache.php
	tests/lib/files/cache/scanner.php
<default></default> and <notnull>true</notnull> contradict each other
Merge branch 'stable5' into fixing-4866-stable5
Merge branch 'fixing-4866-stable5' of github.com:owncloud/core into fixing-4866-stable5
Merge pull request #4957 from owncloud/fixing-4866-stable5

Backport of #4869 to stable5 (recreate an etag within the scanner if the cache contains an empty etag)